The disclosure relates generally to the prevention of damage to masks used in semiconductor manufacturing and, more particularly, to prevention of haze formation on mask surfaces.
Semiconductor manufacturing processes require the creation and maintenance of masks that are substantially free of scratches, particle deposition, and pellicle frames so that each mask is in condition for use in lithography. The presence of defects results in reduced silicon wafer yield during lithography. As such, attention to the prevention and discovery of mask defects is required to prevent yield loss of silicon wafers.
Some mask defects are formed after the lithography process. During the mask making process, masks are rinsed with various compositions, including a composition containing ammonium ions. Residual ammonium ions left on the mask surface react with residual sulfuric acid or sulfate ions from the strip process in a high energy environment. As such, ammonium sulfate [(NH4)2SO4] may form on the mask surface in the presence of water [H2O] under one of the following chemical reactions:H2O+H2SO4+2NH4OH→(NH4)2SO4+H2O2NH3+SO3+H2O→(NH4)2SO4 
Under these conditions, an ammonium sulfate [(NH4)2SO4] haze forms on the mask surface, which impacts the usage life of the mask as well as the yield of silicon wafers.
Current measures known to the inventors for reducing the formation of an ammonium sulfate [(NH4)2SO4] haze on the mask surface include the use of storage cabinets which circulate clean dry air and purge the cabinet with nitrogen. Such cabinets are expensive to obtain and require monitoring.